Industrial plants, such as power plants, steel mills, pulp or paper making plants, have relatively complex water/fluid systems. Organic and inorganic matter deposits on the inner walls of these systems forming an accumulation of fouling or scaling deposits which interfere with the proper operation of the system. This is particularly severe on heated surfaces such as heat exchanger surfaces. This is an unwanted occurrence that causes a number of operational problems such as inadequate heat exchange, plugging of equipment, inefficient usage of chemicals, increased utility costs, lost production due to downtime, corrosion, and downgraded products from increased dirt counts.
In principle, one can distinguish between fouling deposits on the one hand and scaling deposits on the other hand. Fouling deposits are organic deposits which often occur in the form of biofilms in aqueous systems. Such biofilms substantially consist of micro-organisms, e.g. bacteria, algae, fungi and protozoa. Scale is formed from inorganic matter such as complexes of calcium (carbonate, oxalate, sulfate, silicates), aluminum (silicates, hydroxides, phosphates), barium sulfate, radioactive radium sulfate, and silicates of magnesium.
In order to avoid the accumulation of fouling deposits and in particular the growth of biofilms, biocides are added into the fluid concerned as countermeasures. Scale deposits can be removed or prevented by adding chemical deposit control agents based on homopolymers, copolymers and terpolymers of acrylic acid, methacrylic acid, maleic acid and aspartic acid. Chemical deposit control agents include organic phosphonates and their derivatives, as well as polyphosphates. The dosage of these biocides and chemical deposit control agents should be controlled very carefully because they are very expensive.
In line sensors are particularly useful in detecting and quantifying scale for controlling the addition of scale treatment additives. High temperature scaling conditions present a significant challenge to developing an in line scale sensor. Such flows generally preclude the use of most non-metal materials for any surface that is in contact with the flow, and also can be challenging environments for proper operation of electronic components. In addition, the liquid comprising the flow may have other properties that make sensor development difficult; the flow can contain particulates, be toxic, be corrosive to some material, not have constant density, etc. For example, cooling water may have a significant content of dissolved salts, but it is still called water. In many industries, water with a high content of dissolved salts may be called brine, although that term is usually applied to solutions of highly soluble salts. In pulp producing mills, water with certain dissolved salts and dissolved lignin may be called black liquor. Even solutions of highly soluble salts can accumulate troublesome amounts of inverse soluble salts that accumulate in the water for various reasons. Since the most common scale type is inverse soluble scale, the sensor needs to have a surface exposed to the flow that is at a higher temperature than the bulk liquid flow. This requires some form of heating, to produce a heated wetted test surface that is predisposed for the accumulation of scale. The heating must be accomplished in a manner that allows the accumulation to be quantified. This quantification may be a measurement of the reduced heat transfer capabilities resulting from the accumulation, or of the thickness of the accumulation, or both.